1. Field of the Invention
The present invention relates to a hologram recording medium on which information is optically recorded or reproduced, such as an optical disc, an optical card and the like, and more particularly, to a hologram recording medium which has a recording layer irradiated with a light beam for recording or reproducing information, and a recording and reproducing system.
2. Description of the Related Art
The hologram has drawn attention because of its ability to record two-dimensional data signals at a high density, in order to increase the density of recording information. The hologram is characterized by volumetrically recording a wavefront of light which carries recording information on a hologram recording medium made of a photosensitive material such as a photo-refractive material as a change in refractive index. For example, a recording and reproducing system which utilizes the hologram recording medium as a disc (hologram disc) has been developed (see Laid-open Japanese Patent Application Kokai No. 11-311937).
FIG. 1 shows part of a hologram disc. This hologram disc is comprised of a transparent substrate, and a hologram layer, a reflective film, and a protective layer laminated in order on the transparent substrate. The reflective film of the hologram disc has a reflective surface which is formed with servo areas 6 at predetermined angular intervals, and data area 7 in a sector area between adjacent servo areas 6.
In the hologram recording system, a hologram disc with reference light which is converged on the reflective film through the recording layer as a spot, and the reference light reflected by the reflective film diverges to pass through the recording layer, and simultaneously, information light, which carries information to be recorded, is passed through the recording layer. In this way, in the recording layer, the reflected reference light interferes with the information light to form an interference pattern to volumetrically record hologram within the recording layer. The holograms of the interference pattern are recorded in the recording layer adjacent to each other, overlapping in sequence. Then, the reference light is irradiated to detect and demodulate reproduced light restored from each hologram to reproduce recorded information.
In the hologram disc shown in Laid-open Japanese Patent Application No. 11-311937, on which the reference light and information light coaxially impinge from the same side, it is difficult to separate the reference light reflected on the reflective film from the reproduced light from the holograms during reproduction of information. This causes the performance of reading a reproduced signal to be degraded. To solve these problems, an objective lens 12 is immediately preceded by a bisect azimuth rotator 14 which has a pupil divided into two areas, each of which has optical rotating directions different by 90° from each other, as shown in FIG. 1, to prevent the reference light from impinging on a photodetector.
However, the conventional method involves integrally driving the bisect azimuthrotator 14 and objective lens 12, causing an increase in cost. The conventional method also has a problem of a deteriorated recording characteristic from reproduced light corresponding to the vicinity of the division boundary of the bisect azimuth rotator 14.